mtd: spi-nor: Stop passing flash_info around

Some functions called from spi_nor_scan() need a flash_info object.
Let's assign nor->info early on to avoid passing info as an extra
argument to each of these sub-functions.

We also stop passing a flash_info object to set_4byte() and use
nor->info directly.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>